At its peak, the inertial confinement confinement NIF (ICF) burst lasts about 100 trillionths of a second. Broken fuel has a diameter of one hundred million fractions of a meter and is eight times denser than lead. The center of the exploded capsule is several times hotter than the core of the sun.
Developing a clear understanding of what exactly happens during the explosion of NIF under such extreme conditions is one of the biggest problems that researchers face when they work to achieve ignition of thermonuclear fusion on the world's largest laser system with the highest energy.
To help solve this problem, Livermore National Laboratory. Lawrence (LLNL) and its partner laboratories and universities have designed and built an extensive suite of more than a dozen nuclear diagnostic tools, and there will be more in the future.
“What you would like to have in exploding is to know all about exploding plasma,” LLNL physicist Dave Schlossberg said.
“The nuclear diagnostic package is trying to determine various parameters that you can measure independently,” he said. " neutron A visualization system measures the spatial distribution of implosion. Neutron flight time diagnostics measure average energy and drift velocity. And the history of the gamma reaction measures emission over time. By collecting this information, we are putting together the best picture of what is happening in the explosion. "
“Some of the diagnostic“ cross talk ”with each other,” added physicist Kelly Khan. “Some provide different fragments (of information), some have similar fragments, and we can put them all together to make a more complete picture. If you want to achieve ignition, nuclear diagnostics is crucial. ”
Keys to Performance
The key factors determining the effectiveness of implosion are neutron yield, ion (plasma) temperature and a low scattering coefficient — the ratio between the number of high-energy neutrons and lower-energy neutrons scattered as a result of interaction with hydrogen. isotopes in the fuel, an indication of the density of the fuel and the distribution of cold fuel surrounding the hot spot.
Also important are the time of the explosion — the time of peak neutron emission, which characterizes the speed of implosion — and the width of the burn, the time when implosion produces neutrons.
All these and other parameters are evaluated using nuclear diagnostics.
“Nuclear diagnostics is basically the only diagnosis that really measures the density and temperature of the fuel,” said Alastair Moore, head of the nuclear diagnostics group. "And they are absolutely necessary for understanding how well we have collected fuel and how close we are to ignition."
In NIF ICF experiments, up to 192 high-power laser beams heat a cylindrical X-ray “furnace” called a “holraum”. X-rays compress hydrogen, deuterium, and tritium (DT) isotopes, partially frozen inside a tiny capsule suspended inside a hologram. If the density and temperature are high enough and remain long enough, the fuel ignites and generates a self-sustaining thermonuclear reaction that spreads through the fuel and releases a large amount of energy, mainly in the form of high-energy neutrons.
The explosion process creates temperatures and pressures similar to those found inside stars, giant planets and nuclear explosions. The NIF is a key component of the National Nuclear Safety Administration. Inventory management programand NIF experiments promote research in the science of high energy density (HED), including astrophysics, materials science, and ICF.
Of particular value to NIF's nuclear diagnostics is their ability to help answer questions that researchers did not even know what they had – what scientists call "unknown unknowns."
Recently, for example, an array of four neutron time-of-flight detectors located around the target camera showed that a tiny hot spot in the center of the implosion drifts at a speed of about 100 kilometers per second, which indicates the asymmetry of implosion. , the main reason for the decline in performance.
“We initially had two spectrometers,” said physicist Ed Khartuni, “and adding a third spectrometer made it possible for us to see the motion and measure the drift speed of the hot spot, which was not expected at all. In fact, it took quite a while. to be accepted, this interpretation of what these detectors told us.
“They discovered something that happened in the explosion, which we did not expect, which no one expected,” he said. “The fact that the hot spot could move was pretty surprising.”
“Actually, we have a fifth spectrometer,” said Moore, “which will give us an even better opportunity to understand if the hot spot is moving because we are moving asymmetrically, or because the capsule is asymmetric, or hohlraum” is asymmetric. All these types of failures, which can lead to poor implosion characteristics, can be diagnosed directly if several spectrometers look at the same implosion. "
And that's not all. In collaboration with the Los Alamos National Laboratory (LANL) neutron imaging team, researchers from LANL, LLNL and the Laser Energy Laboratory (LLE) at the University of Rochester recently added a third neutron imaging system, NIS3, designed to provide a 3D image showing the size and shape of the burning DT -plasma during the stage of ignition in an explosion.
The size of the hot spot and the asymmetry of the fuel are determined by the image of primary neutrons or neutrons with high energy, and the area of the density of cold fuel, known as rho-R, is determined from the coefficient of low scattering. Area density is an important factor in the final configuration of the fuel to produce ignition and molten combustion.
“As NIF moves toward increased productivity, understanding the three-dimensional nature of these explosions becomes critical,” LLNL physicist David Fittinghoff said. “With the two previous neutron visualization survey lines (at the equator and the north pole of the target camera), we had to make an assumption about the symmetry of the explosion.
“Now with the new NIS3, we have three orthogonal lines of sight with which you can restore the volume of floating plasma,” he said. “An analogy can be the difference between seeing a picture of a person and walking around his sculpture.”
Along with improved neutron imaging, NIS3 also provides a line of sight for visualizing gamma rays resulting from inelastic scattering of thermonuclear neutrons from carbon in the material of the target capsule remaining during the explosion. This can help researchers determine the amount and effect of mixing capsule material with thermonuclear fuel, a known source of performance degradation.
Another important diagnostic update was completed in 2017, when an array of 48 real-time neutron activation detectors, or RT-NAD, was installed at strategic points around the target camera.
Earlier NADs, called flanged NADs, worked when unscattered neutrons activated a zirconium sample. Activated samples were removed from the chamber, and the activation level was determined using nuclear counting methods at another location on the site. Real-time activation of NAD detectors is monitored on site, providing a better sampling of the angular distribution of the unscattered neutron output with much faster turnaround and significantly lower operating costs.
The system provides almost real-time determination of the distribution of neutron fluence. It works with a neutron yield of two to three orders of magnitude, providing a general estimate of the yield with an accuracy of 2 percent or better.
“The neutron yield varies depending on the camera, because you have different thicknesses of fuel in the compressed core of the explosion,” Moore explained. “RT-NAD is primarily a way to find out how fuel is distributed around a hot spot when a capsule explodes.”
“It has twice as many detectors and five times higher sensitivity” of the NAD system with flanges, said physicist Richard Bionta, senior scientist for the RT-NAD system. “In the old system, we had only one detector. Each of the 20 washers was placed in the detector one at a time, so it went through five days. (RT-NAD) is certainly much better than the way. we used to do it. "
“Richard spent over two years developing the ability to manage this data stream,” Moore added. “You have 48 detectors that read every 10 minutes and produce terabytes of data. You are trying to analyze this and again put together the picture that happened with the picture. "
Livermore National Laboratory Lawrence
Nuclear diagnostics help pave the way for fire in the synthesis of inertial confinement NIF (2020, March 13)
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